I didn't do research to see if there is a certain size for minis. I just drew off a piece of w/c paper (WN rough by the way), taped it the size I wanted, and sketched it in. I traced the size of the sign, then free handed the rest..From what little I could see in the photo.
This in no way compares to our miniature painters, but it definitely was NOT easy since I don't have tiny mini brushes.I used the smallest I have but sometimes had to blot off the paint!
I have to admit it was fun to do. I did use a little white gouache.